1. Field of the Invention
The present invention relates to optical communications and, more specifically, to precision mechanical mounting mechanisms for optical components.
2. Description of the Related Art
Wavelength-division multiplexing (WDM) (also known as dense-wavelength-division multiplexing (DWDM)) has been shown as a promising approach for increasing the capacity of existing fiber optic networks. A communications system employing WDM uses multiple optical signal channels, each channel being assigned a particular channel wavelength. In such a WDM system, optical signal channels are typically generated, multiplexed to form an optical signal comprised of the individual optical signal channels, transmitted over a waveguide, and demultiplexed such that each channel wavelength may be individually routed or switched.
One demultiplexing technique involves the use of an optical diffraction grating to separate the wavelengths of an incident optical signal into its constituent channels. Such a technique is described in detail in U.S. Pat. No. 6,307,657 (herein xe2x80x9cthe ""657 patentxe2x80x9d), incorporated herein by reference in its entirety.
As described the ""657 patent, relative alignment of various optical components within the device including the optical grating is important to proper operation of the device. The ""657 patent discusses techniques for relative adjustment of these components, maintenance of reasonable relative alignment between the components over temperature, and aspects of the design that afford tolerance to misalignment. However, as the wavelength spacing between channels in WDM systems decreases, the tolerance of the grating angle with respect to the incident light also decreases. Additionally, as optical components (e.g. micro-mirror arrays) and the devices that incorporate them are made smaller and smaller to accommodate more channels in less space, the linear distance associated with the angular tolerance of the grating location also decreases. These trends lead to the need for increasingly more accurate adjustment and temperature compensation, maintenance, and athermalization mechanisms.
Problems in the prior art are addressed in accordance with principles of the invention by an apparatus and method for enabling coarse and fine alignment adjustment of a diffraction grating within an optical wavelength-division multiplexing (WDM) device and for maintaining the diffraction grating in near-littrow alignment (e.g., to within an accuracy of +/xe2x88x920.003 degrees) over the device""s operating temperature (e.g., xe2x88x925.0xc2x0 C. to 65xc2x0 C.).
In a preferred embodiment, precise grating alignment is maintained in a WDM device by use of a 0.3 parts-per-million per degree centigrade (PPM/xc2x0 C.) Super Invar alloy (e.g., 63Fe-32Ni-5Co) in a wedge-shaped grating mount for the optical grating to eliminate an angular shift of the grating over temperature that is typical in the prior art due to use of other materials such as Invar (e.g., 64Fe-36Ni).
The grating mount is affixed to a first platform whose angle along a first axis is adjustable relative to a second platform. The first platform is attached to the second platform via a first flexure spring oriented parallel to the first axis and the angle is adjusted using first and second adjustment screws. The screws are threaded through the second platform on opposing sides of the first flexure spring and the tips of the screws make contact with the first platform to adjust the angle. The second platform is attached to a base platform (i.e., the grating assembly mounting base) via a second flexure spring oriented parallel to a second axis, the second axis being substantially orthogonal to the urst axis. The angle between the second and base platforms is adjusted using third and fourth screws. In one or more embodiments, adjustment screws are tipped with a jeweled or other low-friction element and mated to their respective platforms via a ball-and-socket arrangement that prevents incremental slippage of the contact over time and temperature and thus prevents minor changes in the angle between the platforms.